Board product and method for making the same

ABSTRACT

A method of forming a coated boxboard product is provided. A boxboard product is formed without being processed by a Yankee dryer, and further comprises a plurality of fiber plies, including outermost plies forming the top and back sides and comprised of bleached chemical pulp, and medial plies disposed between the outermost plies and comprised of groundwood, pressure groundwood, chemithermo-mechanical pulp, recycled pulp, and/or broke. The boxboard product is precalendered with a surface conditioning device and then coated such that the coated boxboard product has a density of between about 500 kg/m 3  and about 1000 kg/m 3 , and a basis weight of between about 150 g/m 2  and about 400 g/m 2 , and the top side has a PPS-s10 roughness of between about 0.5 μm and about 2.0 μm and a Hunter gloss of between about 35% and about 80%.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention relate to a coated board product and a method of producing the same.

2. Description of Related Art

In the field of producing paper products, one ongoing goal is to improve the quality of board products, especially boxboard, and the economy of producing the same.

Board is required to have a certain surface quality for ensuring a desired gloss and print quality, and a stiffness and tear resistance for securing the functionality of a package. Since board is produced in large quantities in a board mill, the efficient use of raw material is also important. However, these demands are somewhat contradictory to each other. Board can be provided with a sufficient gloss by calendering the board by compressing it in a nip, often moistened and heated in a certain manner. The surface fibers and coating of board are preferably pressed smooth by this compression, yet without compacting the middle ply of board. The compaction of a middle ply undermines board stiffness and reduces tear resistance. The compaction of a middle ply is often referred to as a loss of bulk. In this case, bulk is understood as being an inverse value to density and a loss thereof is thus equal to a densifying compaction of paper or board.

Since the process of making paper and board is highly raw material intensive, even a minor saving in raw material provides a major advantage over competitors. In this respect, a saving of just one percent can be considered a major competitive edge and the investment restitution time is short. Saving raw material is also desirable for environmental reasons. By virtue of a reduced weight structure, the multiplicative effects of the board of this invention cover the product's entire life span, the reduced consumption of raw material resulting in a lighter container which ultimately creates savings also in shipping operations and in the way of a reduced amount of waste.

Packing boards are often coated or multi-ply structured. Basic board consists typically of three plies of fiber, wherein the top and back plies are made of bleached pulp. The filler ply consists often of mechanical pulp, typically groundwood (GW), but in many cases also pressure groundwood (PGW) and chemithermo-mechanical pulp (CTMP), or the filler ply can also be made by using broke. The face of board is generally coated twice and the back once. Coating and sizing are used for providing desired properties. A typical basis weight range for boxboards is 180-350 g/m². The necessary basis weight depends on a required stiffness of the container, a lighter board being sufficient for small boxes. Successful conservation of board bulk in surface treatment to produce thereby board of a higher stiffness results in savings of raw material and energy by enabling the use of board of a lesser basis weight. Typical applications for board include cigarette packages, pharmaceutical packages, postcards, cardboard covers for books, various food packages.

Boxboards are often smoothed with a Yankee cylinder prior to coating, which provides a good bulk and stiffness, the surface properties being also good, the drying shrinkage along the edges being likewise small, yet the use of a Yankee cylinder is limited by speed restraint, space demand for equipment and the enormous size of a Yankee cylinder in a high-speed machine. Another typical treatment method involves a wet-stack calender, the drawbacks of which include problems regarding runnability and a controlled application of water and, in addition, extra costs are incurred by the necessity of drying the board before and after a calendering process.

A machine calender is often used together with other calenders, the machine calender referring to a hard calender with no elasticity in its rolls. The use of a machine calender as the sole surface treatment method is not advisable. A soft calender refers to a soft-nip calender, wherein the calender roll has a surface which is elastic, the surface having possibly a hardness in the same order as the surface hardness of wood, yet being elastic.

BRIEF SUMMARY OF THE INVENTION

The above and other needs are met by the present invention which, in one embodiment, provides a method of making a boxboard product having a smooth printing surface, a high gloss and stiffness in the boxboard with a lesser-than-before consumption of material, and avoiding bottlenecks and improving runnability in the production process. In one embodiment, the coated container board of the invention comprises two or more plies of fiber, wherein the outside plies consist of bleached chemical pulp and the inner plies of mechanical pulp or chemithermo-mechanical pulp or broke.

According to the invention, boxboard is treated with a long-nip calender prior to coating or during its coating process in order to upgrade the board qualities over what is known before and, in addition, the production runnability is improved and the production method is not subject to a speed restraint the same way as a Yankee cylinder. A long-nip calender suitable for making a board of the invention has been described, for example, in U.S. Pat. No. 6,164,198 also assigned to the assignee of the present invention.

A calender suitable for the surface treatment of a board of the invention includes a fixed support element, around which is a tubular jacket. A heated counter-element is disposed on the other side of the tubular jacket from the support element, such that a web passes through between said counter-element and the tubular jacket. The fixed support element is provided with load elements, applying the jacket against the heated counter-element and thereby enabling a calendering process between the jacket and the counter-element. The jacket has its opposite ends secured to end walls mounted rotatably relative to the support element, the rotary motion of the end walls being delivered by a separate drive motor, which is independent of a motion of the fibrous web in order to avoid overheating of the jacket.

A method of the invention for conditioning the surface of coated or uncoated board with a surface conditioning device is in turn characterized in that the method comprises feeding a fibrous web through a long nip established by a roll and a counter-roll, the former being in the form of a tubular-shaped flexible jacket. Across the extent of the nip the jacket deflects and thereby presses into contact with the counter-roll over a long stretch. The board treated with the method is lighter than currently available boards, while stiffness and surface properties are equal to those of currently available boards.

The solution enables a running speed substantially higher than what is accomplished with a Yankee-cylinder equipped board-machine. In addition, the runnability is better, this also contributing to improved quality and reducing waste.

Web speed in the calender may be higher than 600 m/min, preferably higher than 800 m/min, and still more preferably 1000 m/min, yet lower than 4000 m/min. Thus, the calender does not restrict the speed of a board machine. The above-mentioned heated roll has a temperature of 150-350° C., preferably higher than 170° C., most preferably about 200-250° C. Linear pressure in the nip is within the range of 100-500 kN/m, preferably less than 400 kN/m, most preferably about 50-300 kN/m. Maximum pressure in the nip is 3-15 MPa, preferably less than 13 MPa, most preferably about 0.5-8 MPa.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:

FIG. 1 is a sectional view of a long-nip calender, provided with a long nip between an enclosed shoe calender and a counter-roll;

FIG. 1A is a partial enlargement of FIG. 1;

FIG. 2A is a partial sectional view of the device shown in FIG. 1, along the roll axis and depicting a drive mechanism; and

FIG. 2B shows the operation of press shoes in a longitudinal section.

DETAILED DESCRIPTION OF THE INVENTION

The present inventions now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the inventions are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.

In FIG. 1, a board web 80 travels through an extended and heated nip 1. The nip 1 is established by an enclosed shoe roll 10 present under the web 80. Above the web 80 is a heatable counter-roll 22. The enclosed shoe roll 10 comprises a flexible jacket 12 impervious to liquid. The jacket consists for example of fiber-reinforced polyurethane. The stationary fixed support element 14 carries at least one load shoe 18. Between the load shoe 18 and the support element is an actuator 20, such as a hydraulic cylinder, for urging the concave load shoe 18 and thereby also the flexible jacket 12 against the counter-roll 22. Thus, the jacket 12 is forced out of its normal unloaded position 11 in a direction away from the center of the enclosed shoe roll. The jacket 12 is fastened at both ends thereof to end walls 24, 26, thus creating a sealed compartment 13 (see FIG. 2). As shown also in FIG. 1, at least one detector device 99 is mounted in communication with the web 80 for detecting web breaks. The detector device 99 is connected to a control device 98 for controlling the operation of a calendering process in dependence of the web being broken or not.

As shown in FIG. 1, the heatable counter-roll 22 is accompanied by a disengagement mechanism, comprising a lever 95 pivotable by a hydraulic cylinder assembly 94 and provided with a pivot point 96 for pivoting the lever thereon. The disengagement mechanism presses the counter-roll 22 to an engagement with the nip 1 and disengages it from the nip 1.

Between the load shoe 18 and the jacket 12 is supplied a pressurized oil, which develops a hydrostatic pressure throughout the nip and presses the jacket to an engagement with the counter-roll 22 over the entire extent of the nip 1. At the same time, the oil protects the jacket from being damaged by lumps and a temperature rise.

In FIG. 2A it is shown that the end walls 24, 26 are rotatably mounted on stub shafts 16, 17 of the support element 14 (The end walls are preferably not integral but divided into a static part and a rotating part as shown in FIG. 2B). On one end of the stub shaft, a cylindrical shaft 32 is arranged rotatably via bearings 34. A support column 36 is arranged to the cylindrical shaft via self-aligning bearings 38, which allow spherical movement to allow the deformation/bending of the support element 14 when heavily loaded. One of the end walls 24 is fixedly attached to the cylindrical shaft. A drive transmission 40 is fixedly attached to the cylindrical shaft outside the end wall, in the shown embodiment a cog wheel. The cog wheel is connected to a transmission 42 and in turn a drive 44. A cog wheel 46 is fixedly attached to the cylindrical shaft inside the end wall. A drive shaft 48 is arranged inside the jacket and parallel to the support element 14. The drive shaft 48 is supported by bearings 50 arranged in bearing houses 52 attached to the support element. At each end of the drive shaft, cog wheels 54 are arranged. Preferably these cog wheels have a prolonged toothed portion to allow axial movement of the intermeshing cog wheel which is attached to the end wall. A further cog wheel 56 is fixedly attached to the second end wall 26 inside the jacket. Both cog wheels inside the jacket mesh with the corresponding cog wheel on the drive shaft. The second end wall 26 is rotatably arranged on the second stub shaft 17. The second stub shaft is in turn fixedly attached to a second support column 58.

The operation is as follows. During normal operation, the driven heated roll 22 is in interaction with the fibrous web and the flexible jacket 12 by a desired pressure being exerted by the load shoe 18, thereby causing a friction based drive of both the fibrous web and the flexible jacket. Accordingly, during normal operation the forces exerted in the nip provide for rotation of the enclosed shoe roll.

Only in specific occasions, it will normally be desirable to operate the independent drive of the enclosed shoe roll 10, for example when starting up the calender. If the calender should be started without first speeding up the flexible jacket 12, this would inevitably cause damage to the flexible jacket due to overheating. Furthermore, it would also be deteriorating for the fibrous web, since at the moment of start it would develop exceptional tension forces in the fibrous web. Accordingly, the independent drive arrangement of the enclosed shoe roll is to be used for instance at the start-up of the calendering surface. At the start, the nip gap is not closed, but the roll 22 has been moved out of contact with the nip 1. Before moving the heated counter-roll 22 into the nip, the drive arrangement 44 of the enclosed shoe roll 10 is activated to accelerate the first end wall 24 via transmissions. The rotation of the end wall causes the inner first cog wheel 46 to rotate, and subsequently the drive shaft 48. The drive shaft transmits the rotation to the second end wall 26 via the second inner cog wheel 56. The both end walls are thus accelerated and rotate at the same speed until a desired peripheral speed is obtained, which is normally equal to the speed of the fibrous web. The nip is closed by activating the hydraulic piston 94 to pivot the lever 95 and thereby moving the counter-roll 22 into the nip and subsequently the load shoe 18 is urged against the heated roll 22 by its actuators 20. Once the calender functions in the desired manner, the drive arrangement of the enclosed shoe roll can be deactivated and the press roll driven in a conventional manner by friction within the nip 1.

In FIG. 2B there is shown an alternative embodiment of the drive arrangement for an enclosed shoe roll. This embodiment uses friction for the transmission of rotational forces.

FIG. 2B also shows a design of arranging the support element and the end walls. The end walls are divided into inner parts 24A, 26A connected non-rotatably to the support element 14, a rotational part 24B, 26B, and a bearing assembly 24C, 26C therebetween. The support element 14 is at its ends arranged with self-aligning bearings 23, 25 to allow a deflection of the support element 14.

In the figure there is shown a drive 44 having a shaft 19B. On the shaft 19B is mounted a disc 19 having a rubber layer at its peripheral end 19A. The outer ends of the flexible jacket 12 are fixedly attached between an annular ring 15, acting as a replaceable force transmitting device, and the periphery of each end wall. The ring 15 is fixedly attached to the end wall. On the inside of the rotational part 24B, 26B of each end wall there is fixedly attached a cog wheel 46, 56. The drive arrangement 44, 19 is movable in and out of contact with the force transmitting device 15. When it is desired to accelerate the enclosed shoe roll 10, the drive arrangement is moved such that the rubber layer 19A comes into frictional engagement with the force transmitting device 15. The cog wheel 46 and the drive shaft 48 transmit the rotation of the end wall 24 to the other end wall 26 by the cog wheels 54, 55 and 56, which at the same time function as a synchronizing device. Hence, both end walls 24, 26 are operated as described in reference to FIG. 2A. FIG. 2B further illustrates in a schematic view one functional embodiment of the load shoe 18. Generally, the load shoe 18 is not disposed diametrically relative to the drive shaft, but perpendicularly as in FIG. 2A.

Tests conducted by the assignee indicated that, in test batches manufactured by a long-nip calender as described above, the board could be provided with a ratio of bulk and smoothness better than in currently available types of board. Thus, according to measurements, the goals of the invention are achieved.

Shoe calenders can be driven at high speeds and, furthermore, by the application of an elevated temperature, e.g. about 250° C., and by taking into account a long dwell time in the calendering zone, the resulting gloss finish will be equal to what is achieved in a slower solution using a Yankee cylinder. In addition, the board is provided with improved bulk. In addition to aspects contributing directly to board quality, the results include savings of production space in a mill, the elimination of a production limiting Yankee cylinder, and the provision of a more manageable, more easily controlled system.

In view of producing board of the invention, surface moistening can be provided prior to calendering. However, the inventive board can also be produced without surface moistening.

Conducted tests showed that better surface properties were obtained for board with equal bulk. Test runs were performed on board which was calendered with the above-described long-nip calender without smoothing it with a Yankee cylinder: Methods Measured For The Same Grade Of Board Precalendering Conditions Board Properties Linear Added Bendtsen PPS Temper- pressure water Bulk roughness roughness Hunter ature ° C. kN/m g/m² cm³/g ml/min μm gloss Yankee reference — — — 1.83 22 1.4 35 Shoe calender 200 100 4 1.84 41 1.5 33 Shoe calender 200 200 4 1.82 25 1.3 32 Shoe calender 250 100 4 1.82 16 1.2 33 Shoe calender 250 200 4 1.82 17 1.2 32 In the test run, reference board and pilot-calendered board were coated twice in a blade coating station, the total amount of coating being about 24 g/m². The products received no final calendering.

Hence, without affecting bulk, the result showed less roughness and more gloss than what was achieved with the Yankee reference. Based on experience, the interpretation of test results represents a progressive step, regarding for example the quality and production economy of boxboard. In general, pilot tests provide results are somewhat less indicative than those achieved in the ultimate environment, so even on the basis of these preliminary tests, it is possible to draw a conclusion that the method is capable of producing board that is better than before and at the same time more easily and economically producible. In addition, the method is applicable to considerably higher speeds than a Yankee cylinder.

Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation. 

1. A method of forming a coated boxboard product, comprising: precalendering a boxboard product with a surface conditioning device comprising a heatable counter-roll disposed adjacent to a tubular flexible jacket extending around a fixed support element and having a load element disposed therebetween for biasing the flexible jacket against the counter-roll, the flexible jacket having opposed ends and being mounted to at least one end wall at each end, the flexible jacket and the at least one end wall at each end being rotatingly driven by a drive mechanism operably engaged therewith, the boxboard product being directed between the flexible jacket and the counter-roll so as to be precalendered thereby, the boxboard product having a top side, a back side, and being formed without being processed by a Yankee dryer, the boxboard product further comprising a plurality of fiber plies, including outermost plies forming the top and back sides and comprised of bleached chemical pulp, and medial plies disposed between the outermost plies and comprised of at least one of groundwood, pressure groundwood, chemithermo-mechanical pulp, recycled pulp, and broke; and coating the boxboard product following precalendering thereof such that the coated boxboard product has a density of between about 500 kg/m³ and about 1000 kg/m³, and a basis weight of between about 150 g/m² and about 400 g/m², and the top side of the coated boxboard product has a PPS-s10 roughness of between about 0.5 μm and about 2.0 μm and a Hunter gloss of between about 35% and about 80%.
 2. A method according to claim 1, wherein coating the boxboard product further comprises coating the top side at least once.
 3. A method according to claim 1, wherein coating the boxboard product further comprises coating the boxboard product without coating the back side thereof.
 4. A method according to claim 1, wherein coating the boxboard product further comprises coating the back side at least once.
 5. A method according to claim 1, wherein coating the boxboard product further comprises coating the boxboard product in a coating process such that the coated boxboard product has a basis weight of between about 180 g/m² and about 350 g/m².
 6. A method according to claim 1, wherein coating the boxboard product further comprises coating the boxboard product in a coating process such that the coated boxboard product has a basis weight of between about 180 g/m² and about 300 g/m².
 7. A method according to claim 1, wherein coating the boxboard product further comprises coating the boxboard product such that the top side thereof has a Bendtsen roughness of between about 0 ml/min and about 50 ml/min.
 8. A method according to claim 1, wherein coating the boxboard product further comprises coating the boxboard product such that the top side thereof has a Bendtsen roughness of between about 0 ml/min and about 20 ml/min.
 9. A method according to claim 1, wherein coating the boxboard product further comprises coating the boxboard product such that the top side thereof has a PPS-s10 roughness of between about 0.8 μm and about 1.5 μm.
 10. A method according to claim 1, wherein coating the boxboard product further comprises coating the boxboard product such that the top side thereof has a Hunter gloss of between about 40% and about 65%.
 11. A method according to claim 1, wherein coating the boxboard product further comprises coating the boxboard product such that the coated boxboard product has a density of between about 750 kg/m³ and about 1000 kg/m³.
 12. A method according to claim 1 further comprising calendering the boxboard product with a calender following precalendering of the boxboard product, the calender being selected from the group consisting of at least one nip and a soft calender.
 13. A method according to claim 1, wherein precalendering the boxboard product further comprises moistening at least one of the top side and the back side of the boxboard product.
 14. A method according to claim 1, wherein precalendering the boxboard product further comprises precalendering the boxboard product without moistening either side thereof. 